Such a packaged resonator of small dimensions has been disclosed in the prior art document U.S. Pat. No. 7,084,556 filed in the name of the Assignee, and which is incorporated herewith by way of reference. Resonator 10, represented on FIG. 1, is intended to be mounted in a package. It includes a tuning fork shaped part with two parallel arms 12, 14 connected to each other by a linking part 16 and carrying electrodes 20, 22 to make them vibrate, these electrodes being connected to connecting pads 28, 30 intended to be electrically connected to the exterior of the case.
FIG. 2A is a top view of a known package (shown with its lid off) for receiving a piezoelectric resonator of the type corresponding to the field of the invention. It is understood that the size of the package depends on the general shape of the resonator, which may vary according to the embodiment considered. Package 70 of parallelepiped shape includes a case formed by a flat bottom 72 and four sides 74, and a lid (not shown). Two conductive elements, such as thin conductive layers, studs or bumps 78 and 80, are arranged on the flat bottom 72 for contacting the connecting pads 28, 30 of the resonator 10 (FIG. 1). After the resonator 10 has been mounted in the case 70, the lid can be fastened on by vacuum soldering its edges onto the sides 74 of the case. This can be done by heating and pressure, by means of a soldering frame (not shown). In a known manner, the flat bottom 72 and the four sides 74 of the case of FIG. 2A can be formed by etching layers of ceramic. If the lid for the case is not made of ceramic like the rest of the package, it can be made for example from metal, glass or silicon.
One of the problems with most conventional resonators is that they are designed to be bonded to a support substrate at one of their ends only. With such a design, it is difficult to guarantee that the planar tuning-fork-shaped part of the resonator is properly positioned and aligned parallel to the surface of the support substrate. In case of misalignment, there is a risk that the vibrating arms of the resonator will collide with the surface of the support substrate or with the lid of the package. In order to prevent such an outcome, sufficient clearance must be provided above and below the resonator. This requirement for clearance severely limits the possibility of designing ever thinner and more compact packaged piezoelectric resonators.
Prior patent document U.S. Pat. No. 7,084,556 partly solves the above problem by providing an improved tuning fork resonator including an additional central attachment arm 18 (FIG. 1) connected to linking part 16 and located between arms 12, 14 of the tuning fork shaped part, substantially equidistant from them. As depicted in FIG. 1, a base part 62 can further be formed at the end of the attachment arm opposite the linking part 16. According to the depicted example, the base part extends beyond vibrating arms 12 and 14 and preferably carries the connecting pads 28, 30 of the resonator. The resonator 10 is mounted in its case by bonding the base part 62 and/or the attachment arm 18, through at least one attachment point, to the bottom of the case. The base part is preferably used both for bonding resonator 10 to the inside its packaging, and for electrically connecting electrodes 20, 22 to conductive elements inside the packaging (not shown).
The provision for a resonator with at least one attachment arm disclosed by patent document U.S. Pat. No. 7,084,556 amounts to a considerable improvement. However, a need subsists for an even slimmer and more compact packaged piezoelectric resonator. Therefore, one aim of the present invention is to further improve the slimness and the compactness of packaged piezoelectric resonators.
One way of achieving this result is to reduce the thickness of the lid of the package housing the piezoelectric resonator. However, the inside of the package is usually maintained under vacuum conditions. Therefore, the lid is submitted to considerable pressure exerted by the atmosphere on the outside of the package. In many cases, the atmospheric pressure will be enough to cause the lid to sag substantially. As the clearance between the vibrating arms and the lid should be minimal, this sagging of the lid can cause collisions between the vibrating arms and the underside of the lid.